A preferred gain-medium for high-power DPSS lasers is neodymium-doped yttrium orthovanadate (Nd:YVO4). This material can be optically pumped at several diode-laser wavelengths, including 808 nanometers (nm), 816 nm, and 880 nm. Longitudinal-pumping (end-pumping) is a preferred pumping method. For DPSS lasers having a fundamental output greater than about 1 Watt (W), the pump-light is usually supplied by an array of diode-laser emitters.
In end-pumping arrangements, light from individual diode emitters is collected by an optical fiber array, which is formed into a bundle. Light from the fiber bundle is usually collimated and focused into the gain-medium by one or more lens elements. Sometimes, an additional multi-mode optical fiber is used to transport the pump beam from the fiber bundle to the collimation and focusing optics. Light from the diode-emitters is strongly plane polarized, but becomes substantially depolarized during transmission through the fiber array and any transport fiber. This creates a problem for optimizing the pumping of Nd:YVO4 when the absorption of pump-light is polarization sensitive, which is the case for many of the preferred pump-light wavelengths. The absorption spectrum of Nd:YVO4 and other polarization sensitive gain-media is usually described in terms of pi (π) and sigma (σ) components, representing absorption in two orthogonal polarization orientations corresponding to crystal axes of the gain-media.
By way of example, FIG. 1 schematically illustrates the pi and sigma polarized absorption spectra of Nd:YVO4 at wavelengths between 725 nm and 900 nm. The peak absorption at 808 nm is about 4 times stronger in the pi polarization than in the sigma polarization. The peak absorption around 880 nm is at least 7 times stronger in the pi polarization than in the sigma-polarization.
In gain-media with polarization-dependent absorption spectra there may be discrete wavelengths at which these spectra intersect, i.e., wavelengths at which the pi and sigma absorptions are equal. Pumping at these wavelengths can be optimized for partially polarized or un-polarized pump-light. Such a pumping scheme is described in U.S. Pat. No. 6,898,231 assigned to the assignee of the present invention. In this scheme, Nd:YVO4 is pumped at a wavelength of 816 nm, a wavelength at which the pi and sigma absorption spectra of the material intersect. In a paper “Optimized Pumping of NdYVO4 with Polarization Insensitive Absorption for High-Power Applications”, McDohough et al., Paper No. CMS5, Proceedings of CLEO 2005, published by the Optical Society of America, a scheme is described where Nd:YVO4 is pumped at a wavelength of 888 nm, which is another wavelength at which the pi and sigma absorption spectra of the material intersect.
One problem with such pumping schemes is that these discrete wavelengths do not occur at peaks in the absorption spectra and the absorption is relatively weak. This can be compensated by increasing gain-medium length, however, this leads to difficulty in mode-matching the pump beam to the lasing mode in the gain-medium. This can be overcome by using a relatively high doping concentration. However, high doping concentrations can lead to reduced optical-pump to optical-output efficiency due to inter-ion energy-transfer processes, such as energy upconversion and cross-relaxation quenching.
In order to optimize the optical to optical efficiency, essentially all the pump-light delivered to a gain-medium should be absorbed by the gain-medium. To achieve this with a partially-polarized or unpolarized-light pump beam, at a wavelength where the pi and sigma absorptions are substantially different (which is usually the case at absorption peaks) the doping concentration of the gain-medium and the length of the gain-medium need to be selected so essentially all of the pump-light in the weakly-absorbed polarization is absorbed over the full length of the gain-medium. This would, however, mean pump-light in the strongly-absorbed orientation would be absorbed over a comparatively short length of the gain-medium. This produces significant local heating. The resulting temperature gradients can cause aberrations in thermal lensing of the gain-medium, and can be sufficient to crack the gain-medium. There is a need for a more reliable method of pumping a polarization-sensitive gain-medium with partially polarized or unpolarized light at wavelengths corresponding to the peaks in the absorption spectrum of the gain-medium.